Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
The operating temperature of the fuel cell is high, about 800° C. or more. Therefore, if a fuel gas consumed in the fuel cell, containing unburned gas discharged to the area around the fuel cell is mixed with an oxygen-containing gas to burn the unburned gas, the temperature of the fuel cell stack may increase significantly. In this case, the operation of the fuel cell stack cannot be performed stably.
In an attempt to address the problem, for example, Japanese Laid-Open Patent Publication No. 5-41241 discloses a solid oxide fuel cell as shown in FIG. 16. In a heat insulating container 1 of the fuel cell, a stack chamber 3 containing stacks 2, an exhaust gas combustion chamber 4, and a heat exchanger chamber 5 containing a heat exchanger 5a are provided. An exhaust fuel gas and an exhaust oxygen-containing gas flow into a fuel gas discharge chamber 3a and an oxygen-containing gas discharge chamber 3b, and the fuel gas discharge chamber 3a and the oxygen-containing gas discharge chamber 3b are provided hermetically from each other in the stack chamber 3. The fuel gas discharge chamber 3a and the oxygen-containing gas discharge chamber 3b are connected to a combustion chamber 4 through exhaust gas passages 6a formed in a heat insulating wall 6.
In the conventional technique, the fuel gas and the oxygen-containing gas heated to the temperature about 700° C. to 900° C. by the heat exchanger 5a are supplied respectively to the stacks 2, and consumed in the power generation reaction in the stacks 2.
The exhaust fuel gas and the exhaust oxygen-containing gas discharged from the stacks 2 to the fuel gas discharge chamber 3a and the oxygen-containing gas discharge chamber 3b flow into the combustion chamber 4 through the exhaust gas passages 6a. Therefore, in the combustion chamber 4, the exhaust fuel gas and the exhaust oxygen-containing gas are mixed together, and burned. The burned gas is supplied to the heat exchanger 5a, and heat exchange with the fuel gas and the oxygen-containing gas before consumption is carried out. Then, the burned gas is discharged to the outside.
However, in the conventional technique, the mixed exhaust gas of the exhaust fuel gas and the exhaust oxygen-containing gas discharged from the stacks 2 has a considerably high temperature. The unburned hydrogen in the mixed exhaust gas is afterburned in the combustion chamber 4, i.e., some of the hydrogen which has not been burned in the stacks 2 is burned after it is discharged from the stacks 2 to further increase the temperature of the mixed exhaust gas. Therefore, the temperature of the combustion chamber 4 is considerably higher than the operating temperature of the stacks 2. The combustion chamber 4 needs to be fabricated uneconomically using expensive heat resistant alloy or the like.
Further, the mixed exhaust gas having the considerably high temperature is supplied to the heat exchanger chamber 5, and heat exchange between the mixed exhaust gas and the oxygen-containing gas and the fuel gas before consumption is carried out. Therefore, in order to make it possible to exchange sufficient heat energy between the mixed exhaust gas having the high temperature and the reactant gases (oxygen-containing gas and fuel gas) having the low temperature, the heat exchanger 5a needs to be considerably large. Further, since the heat exchanger 5a is exposed to the mixed exhaust gas having the high temperature, the heat exchanger 5a needs to be fabricated using expensive heat resistant alloy. Accordingly, the overall size of the fuel cell is large, and the fuel cell is fabricated uneconomically.